Bicycle Mechanics - Why can't I hit the low or high gears?

I'm really having some troubles here getting my lowest gear (small front/big back) and highest gear (big front/small back) to run smoothly! I've been playing with the dang barrel adjusters for about an hour and a half now, and when it's on both these two gears, the chain is rubbing against the front derailer. It is rubbing against the inside part of the cage for the lowest gear, and rubbing against the outside part of the derailer in the highest gear. Should I start loosening the limit screws? The last thing I want is to damage the bike in any way. The more detailed the info the better. Thanks for your help, and I apologize for all the posts.

Here's a couple links with instructions on setting up derailleurs.
Park Tools site (http://www.parktool.com/repair/byregion.asp?catid=53)
And Sheldon Brown's site (http://sheldonbrown.com/derailer-adjustment.html)

Hope one or both help.

Sounds like a limit screw problem to me.

No apologies needed. Posting is what makes it a "forum"!

Another thing to remember, these two gear combinations are not frequently used. Assuming that the derailleurs are set correctly, there may be inherent chainline issues with the frame, which contributes to the problems you are experiencing.

Another thing to remember, these two gear combinations are not frequently used. Assuming that the derailleurs are set correctly, there may be inherent chainline issues with the frame, which contributes to the problems you are experiencing. The highest and the lowest gear should always be accessible on a bike. The only combos in which chain rubbing is acceptable are cross-chaining combos (big chainring-big cog and small chainring -small cog) since those should not be used anyway.

Sounds like a limit screw problem to me.

+1......in the gear combos you mention, adjust the limit screws until there's a small gap between the derailleur cage and chain. Easy fix-

I think you're thinking bib:big and small:small, whereas the OP was asking about small:big and big:small, which should never have chainline issues.

I think you're thinking bib:big and small:small, whereas the OP was asking about small:big and big:small, which should never have chainline issues.

I take it you're addressing "tkncwa."

Yup. I should have quoted. And you're right, it sounds like a limit screw issue.